The use and configuration of bleed valves are well known in gas turbine engines and are usually used to improve engine operability particularly for the engine's compressors. In use heated air at high pressure passes from a compressor, through a bleed valve and via a diffuser into a main gas stream. The compressor may be either an intermediate or high pressure compressor with temperatures of exhausted gas may be up to 400° C. The diffuser is usually a domed plate comprising an array of holes to enhance mixing with the main gas stream, which is usually a cooler bypass flow.
Known bleed valve diffusers, such as installed on the Trent 500 aeroengine of Rolls-Royce™ plc, are designed with the intent to a) attenuate noise produced within the bleed valve; b) produce small separate jets of bleed air (rather than one large one) to increase the jets noise frequency, which is better attenuated within the bypass duct and atmosphere; and c) improve mixing of the hot gases flowing through the bleed valve with the cold bypass flow in order to limit/prevent thermal damage to nacelle and other components.
These diffusers are usually circular domes and have an array of holes where each hole is angled radially so that its air flow jet has both radial and axial velocity components relative to the diffuser's centre-line. Some diffusers, like the Trent 500's, have a number of zones of holes where each zone's holes have different axial angles to the centre-line. Each of these diffusers attempts to spread the flow by pointing each hole (or group of holes) in different directions. This is intended to enhance the mixing of the hot gas passing through the diffuser with the cool bypass flow to reduce its temperature before impinging on thermally sensitive nacelle and/or engine parts. However, it has been found that these prior art diffusers are ineffective because the individual flows from each small hole do not act independently. Although each small gas stream is initially discrete and is directed in a slightly different direction, within a short distance they conglomerate into a single plume. This is because the bypass flow is unable to achieve significant penetration into the mass of individual plumes and so a partial vacuum forms between them. This vacuum causes the plumes to turn until they are pointing in the same direction and thus forms a single plume entity.
Increasing the number of bleed valve assemblies could improve mixing overall, however, this adds significant weight, cost and takes up more space.